1. Field of the Invention
The present invention relates to a formation of metallic wirings in manufacturing highly integrated silicon semiconductor elements, and more particularly to a method for depositing a tungsten nitride thin film using feed gases of WF.sub.6, NH.sub.3 and H.sub.2 with a plasma-enhanced chemical vapor deposition process, prior to a formation of a tungsten thin film using feed gases of WF.sub.6 and H.sub.2 with a plasma-enhanced chemical vapor deposition process, for restraining an occurrence of corrosions at a silicon substrate, an oxide film and boundary surfaces thereof, during the formation of tungsten thin films.
2. Description of the Prior Art
Fabrication techniques for highly integrated circuit elements have made remarkable progress for the last years and recently come to improve the integration degree up to a mega-bit level. Presently, researches for further improving the integration degree are actively advancing. In particular, it is known that metallic wiring formation, that is, metallization is the most important technique for providing improvements in signal processing rate and integration degree in highly integrated circuit elements of 4 mega-bits grade.
Conventionally, metallic wirings used in fabricating highly integrated circuit elements are made of aluminum or polycrystalline silicon thin films. In cases of aluminum thin films, they are formed using a physical deposition process, thereby exhibiting poor step coverage and electromigration characteristics. Upon alloying with silicon of substrates, these aluminum thin films also cause junction spikes which result in degrading yield and reliability of semiconductor elements.
In cases of polycrystalline silicon thin films, they have no problems involved in the aluminum thin films and thus exhibit superior characteristics in terms of reproductiveness and reliability. However, they have a fatal disadvantage that improvements in signal processing rate and integration degree are hardly expected, because of exhibiting a high resistivity.
On the other hand, recently developed metallization techniques include a method of depositing tungsten thin films using a low pressure chemical vapor deposition. Tungsten thin films obtained by this method exhibit a high thermal stability, by virtue of having a melting point of very high level, for example, 3,370.degree. C. They also have the resistivity similar to that of aluminum thin films, thereby exhibiting a superior electromigration. They also have an advantage of improvements in step coverage and signal processing rate, in that the deposition thereof is achieved with the chemical vapor deposition process.
For the deposition of tungsten thin films using the chemical vapor deposition process as mentioned above, WF.sub.6 -H.sub.2 reactant systems or WF.sub.6 -SiH.sub.4 -H.sub.2 reactant systems are mainly used as reactant gas mixtures. During a reducing reaction of WF.sub.6 in a reactant gas mixture, fluorine atoms are generated from the WF.sub.6 feed gas which causes corrosions at a silicon substrate, an oxide film and boundary surfaces thereof by the reaction between the fluorine atoms and silicon. Such corrosions result in penetration of tungsten into the boundary surface between the silicon substrate and the oxide film or into the silicon substrate, thereby causing various problems of high leakage current, short circuiting and low insulation breakdown voltage.
For solving the problems resulting from the corrosions caused by fluorine, there has been proposed a method in which relative partial pressure ratios of SiH.sub.4 and H.sub.2 to WF.sub.6 are properly adjusted to restrain the formation of fluorine while the deposition rate is increased to make a tungsten thin film itself function as a protection film for shielding fluorine to prevent it from reacting with the silicon substrate or the silicon oxide film. At some deposition temperatures and some compositions of reaction gas mixtures, however, this method loses the reliability (reference: R. V. Joshi, K. Y. Ahn, and P. M. Fryer, Proc. of the 1988 Workshop on Tungsten and Other Refractory Metals for VLSI Application IV, Material Research Soc., Pittsburgh, P.85, 1988).
To shield fluorine atoms generated from WF.sub.6 in advance for preventing it from reacting with the silicon substrate or the silicon oxide film will be the ideal method capable of restraining the occurrence of corrosions at the silicon substrate, the oxide film and boundary surfaces thereof during the chemical vapor deposition of tungsten thin film, irrespective of relative composition ratios of SiH.sub.4 and H.sub.2 to WF.sub.6 and deposition temperatures.
Referring to recently published literatures (for example, Silicon Processing for The VLSI Era, Ed. S. Wolf, R. N. Tauber, Lattice Press, Sunbeach, P.556, 1986), it is reported that upon comparing the etch rate of silicon oxide film with the etch rate of silicon nitride thin film by utilizing NF.sub.3, the silicon nitride thin film is slower than the silicon oxide film by about 8:1. This means that an insulating thin film containing nitrogen atoms can not be easily etched out by the fluorine based etching gas. Therefore, if NH.sub.3 gas in addition to the feeding gases of WF.sub.6 and H.sub.2 radicals, NH radicals will react with WF.sub.6 and silicon surface and the binding structure of tungsten-nitrogen-silicon dangling bond on the bare silicon surface of silicon substrate should be formed. As a result, tungsten nitride thin film formed on the bare silicon surface prevent the reaction of fluorine atoms with bare silicon surface directly.
Based on this fact, the inventors found that a proper utilization of a tungsten thin film containing nitrogen exhibiting a property of protecting the tungsten layer from the penetration of fluorine atoms prior to the reaction of fluorine atoms and the silicon surface would make it possible to prevent the corrosions of the silicon substrate and the oxide film during the chemical vapor deposition of a tungsten thin film. In view of this point, the present invention has been made through various experiments of forming tungsten nitride thin films on silicon wafers.
The process of growing a tungsten nitride thin film on a silicon wafer involves a process which precedes the deposition of tungsten thin films for subsequent metallization. The tungsten nitride thin film grown with the process should not affect the intrinsic function of tungsten thin films as metallic wirings.
However, preceding simple coating of thin films containing nitrogen disables the formation of metallic wirings made of tungsten exhibiting a low resistivity, since most of thin films containing nitrogen are formed into insulation thin films.